1. Field
The present disclosure relates to a light-emitting device, and more particularly, to a method and an apparatus for light-emitting device arrays.
2. Description of Related Technology
A person skilled in the art will appreciate that the concepts disclosed herein are applicable to packages for semiconductor-based light-emitting device, namely a light-emitting diode (LED) device.
LEDs have been used for many years in various light requiring applications, e.g., signaling states for devices, i.e., light on or off, opto-couplers, displays, replacement of bulbs in flashlights, and other applications known in the art. Consequently, LEDs emitting both spectral colors and white light have been developed. There are two primary approaches to producing light with desired properties using LEDs. One is to use individual LED dice that emit the three primary colors—red, green, and blue, and then mix the colors to produce light with the desired properties. The other approach is to use a phosphor material to convert monochromatic light from a blue or ultra-violet color emitting LED die or dice to a light with the desired properties, much in the same way a fluorescent light bulb works. For the purposes of this disclosure a die has its common meaning of a light-emitting semiconductor chip comprising a p-n junction.
Due to LEDs' advantages, i.e., light weight, low energy consumption, good electrical power to light conversion efficiency, and the like, an increased interest has been recently focused on use of LEDs even for high light intensity application, e.g., replacement of conventional, i.e., incandescent and fluorescent light sources, traffic signals, signage, and other high light intensity applications known to a person skilled in the art. It is customary for the technical literature to use the term “high power LED” to imply high light intensity LED; consequently, such terminology is adopted in this disclosure, unless noted otherwise. To increase intensity of the light emitted by the light-emitting device, often more than one light-emitting die is arranged in a package; such a light-emitting device being termed a light-emitting device array. For the purposes of this disclosure, a package is a collection of components comprising the light-emitting device including but not being limited to: a substrate, a die or dice (if an array), phosphors, encapsulant, bonding material(s), light collecting means, and the like. A person skilled in the art will appreciate that some of the components are optional.
A conceptual structure of an exemplary light-emitting device array 100 in accordance with known concepts is depicted in FIG. 1a. A substantially flat substrate 102 in addition to being a mechanical support for the electrical and optical layers of the light-emitting device is often used as means for heat dissipation from the light-emitting device array. The electrical and optical layers comprise all the components of the package, excluding the substrate 102. When used as means for heat dissipation, the substrate 102 is made from a material with high thermal conductivity. Such material may comprise metals, e.g., Al, Cu, Si-based materials, ceramics such as AlN, or any other material whose thermal conductivity is appropriate for the light-emitting device array in question. A person skilled in the art will appreciate that material appropriate for a light-emitting device array with power dissipation of, e.g., 35 milliwatts (mW) is different than material appropriate for a light-emitting device array with power dissipation of, e.g., 350 mW. A material is considered to be substantially flat if the irregularities in flatness would not cause light to be reflected by such irregularities.
The source of light comprises a plurality of dice 114 (three dice shown), disposed on an upper face 104 of the substrate 102. A person skilled in the art will appreciate that the number of dice is a design decision, and different number of dice can be used to satisfy design goals.
To improve light extraction from the light-emitting device array 100, several measures are taken. First, surfaces that are transparent to photons emitted at a particular wavelength or that have poor reflectivity of such photons in an undesirable direction of emission may be treated, e.g., by polishing, buffing, or any other process, to acquire a specific reflectivity. Reflectivity is characterized by a ratio of reflected to incident light. Such surfaces are an upper face 104 of the substrate 102 and inner wall 106 of a support member 108. The support member 108 provides boundary for an encapsulant 110 and reflects light emitted by the dice 114 into desirable direction. Alternatively, the desired reflectivity is achieved by applying a layer of a material with high reflectivity, such as Ag, Pt, and any like materials known to a person skilled in the art, (not shown in FIG. 1) onto such surfaces.
Furthermore, to prevent reflection of the emitted photons from boundaries between materials characterized by different refraction indexes, and, consequently, loss of light intensity, an encapsulant 110 is applied into a cavity 112, surrounding the light-emitting region, i.e., the cavity created by the substrate 102, the support member 108, and the dice 114. The material for the encapsulant 110 is selected to moderate the differences between the refraction indexes of the materials from which components creating the reflective boundaries are made. In one aspect of the disclosure the encapsulant 110 is transparent; however, the disclosed concepts apply equally to encapsulant 110 comprising fillers, e.g., phosphors used for light conversion as described above.
Additionally, light-emitting device array package may further comprise a cover 116 disposed above the dice 114. Such a transparent cover comprises e.g., a window or a lens. In order to prevent delamination of the encapsulant 110 from the surface of the cover 116 and/or the inner wall of the support member 108 and/or the dice 114 and/or the substrate 102, the cover 116 is allowed to float freely on the encapsulant 110, without being rigidly anchored onto the support member 108 with an adhesive or another fastening means. Such a configuration prevents significant residual stress, caused by temperature variation as the light-emitting device array 100 heats and cools during the device's lifetime, to develop within the encapsulant 110. Because any delamination would introduce voids in the encapsulant, the resulting internal reflection optical losses caused by the above-described difference between materials characterized by different refraction indexes would cause loss of light intensity.
Although the configuration depicted in FIG. 1 may be suitable for LED packages comprising a clear cover, it is not particularly suitable for LED package comprising a window or lens coated with or filled with phosphors; such a cover being often used for light conversion. An advantage of such a configuration is that the window or lens coated with or filled with phosphors can be matched appropriately with a LED dice of known wavelength to achieve a more precisely controlled color corrected temperature (CCT). Different windows or lenses may have different phosphor coatings or fillings, and these matched with LED dice of optimal wavelength to achieve target CCT as needed.
However, a problem with this configuration arises from the fact that the temperature of the phosphor coated or filled cover increases significantly during operation of the light-emitting device array because the conversion inefficiency of the phosphors results in generating significant heat. The increase in the temperature in turn results in decreased efficiency of the light-emitting device array due to decrease in light-conversion efficiency of the phosphors and decrease of efficiency of the die.
The above-described problem may be solved by a configuration according to FIG. 2, which depicts a conceptual cross section of another exemplary light-emitting device array 200 in accordance with known concepts. The description of like elements between FIG. 1 and FIG. 2 is not repeated, the like elements have reference numerals differing by 100, i.e., reference numeral 102 of FIG. 1 becomes reference numeral 202 in FIG. 2.
Referring to FIG. 2, the main conceptual difference from FIG. 1 is that a cover 216 coated with or filled with phosphors is attached to the upper face 218 of the thermally conductive support member 208. The bottom face 220 of the support member 208 is attached to a thermally conductive substrate 202. Thus, in this aspect, the support member further serves as supporting means for the cover 216. The cover 216, the support member 208, and the substrate 202 should be attached to one another using any thermally conductive means (not shown in FIG. 2) to maximize heat transfer between these components. By the means of example, such a thermally conductive means may comprise material such as metal filled epoxy, eutectic alloy, and any other thermally conductive means known to a person skilled in the art. Furthermore, it is desirable that the cover 216 is also made from a thermally conductive martial. Such a configuration allows heat to flow from the phosphors through the window or the lens 216 and then through the support member 208 to the substrate 202.
Since additional heat from the cover 216 is now transferred to the substrate 202, proper heat dissipation from the LED package 200 must be assured to prevent loss of efficiency due to increased temperature of the dice 114. Such heat dissipation may be achieved by proper design of the above-described components of the LED package 114. In addition, the LED package 200 may further be attached to a suitable heat sink (not shown).
In any of the above-described configurations, the LED package 200 can operate without the phosphors or the LED dice over-heating beyond temperature that would significantly decrease the efficiency of the LED dice and the phosphors. A person skilled in the art will appreciate that the term significant describes a decrease in efficiency that would cause the light-emitting device array performance fail to meet typical or minimum specification over the product life of the light-emitting device array.
The above-described structures of a light-emitting device array suffer from several shortcomings. The light-emitting device design goal determines geometry of the light-emitting device package, which in turn determines the required quantity of phosphor. Thus, any decrease in the quantity of phosphor would improve economics of production. Additionally, the geometry of the package determines a contact area between the phosphor and the substrate, which is subject to a chemical reaction between the phosphor and substrate, resulting in, e.g., tarnishing, discoloration, and the like, of the substrate. Thus, any decrease of the contact area would decrease such undesirable effect, thus improving reliability. Furthermore, the light efficiency is limited by a light cross-talk, i.e., a condition when a light emitted by one of the plurality of dice is absorbed by one or more other dice of the plurality of dice.
Accordingly, there is a need in the art for a light-emitting device array providing solution to the above identified problems, as well as additional advantages evident to a person skilled in the art.